The present invention relates to a short focal length imaging system and, more particularly, to a lens assembly incorporating a plurality of rows of lenses, each lens row associated with a different magnification value.
The increasing use of compact, desk-type electrophotographic document reproduction machines has at least partially been enabled by the use of optical imaging systems having relatively short total conjugates; e.g. under 100 mm. One example is the use of a strip lens in an imaging system of the type described in U.S. Pat. Nos. 3,584,950 and 3,584,952. As shown in these patents, three lens strip elements, each element containing a plurality of refracting lenslets, are coaxially aligned to form a three lenslet assembly having a total conjugate of approximately 64 mm. With this lens design, exact alignment of the lenslets to achieve a 1:1 erect imaging on a photosensitive surface has proven difficult to achieve. The strip lens design also requires additional optical baffles between lenses to eliminate crosstalk.
A second example of a short focal length imaging device is disclosed in U.S. Pat. Nos. 3,947,106 and 3,977,777. As disclosed in these patents, a plurality of gradient index optical fibers are bundled together to form a linear projection lens. This gradient index lens array, known as a SELFOC lens (mark registered in Japan and owned by Nippon Sheet Glass Co., Ltd.) has found use in a number of commercial document reproduction devices as a replacement for the conventional lens/mirror optical systems.
A third example of a short focal length imaging system is disclosed in EPO Publication No. 0133788 published on Mar. 6, 1985 and in U.S. Pat. No. 4,488,864. The lens array of this system comprises a photosensitive soft glass which is exposed, on opposed surfaces, to collimated ultraviolet light through an apertured mask. Following a subsequent heat treatment, small, spherical thick lenses are formed at the surfaces of the glass, the lenses arranged in a row to create a lens array that functions as an image focusing device.
The prior art described in the above-identified patents discloses various embodiments of lens arrays which project erect images, at unity magnification, of documents when the lens is placed along an optical axis between a document and an image plane. It is known, according to U.S. Pat. No. 4,331,380, that a gradient index lens array can be designed so as to enable image projection at magnification ratios other than unity. In the '380 patent, there is further disclosed a particular document imaging embodiment wherein a conventional 1:1 gradient index lens array is combined with a reduction/enlargement gradient index lens array to form a lens assembly which, depending on its positioning in the optical path, reproduces a document at either 1.times. magnification or at some reduced or enlarged magnification. It is further known, through copending application Ser. No. 839,667 filed on Mar. 14, 1986 that a thick lens array of the type described in the EPO publication can be formed so as to enable a thick lens type of lens array to have magnification properties. Application 839,667, in its entirety, is hereby incorporated by reference.
These prior art lens arrays, except for the array disclosed in the '380 patent, are limited in that they do not provide a user with more than one magnification option. It would be desirable to utilize a short focal length lens assembly which incorporates, within a common structure, a plurality of lens arrays each lens array associated with a different magnification. The present invention is therefore directed to an optical system for an electrophotographic reproduction machine including a short focal length lens assembly for projecting an image of a document at an object plane along an optical path onto a photosensitive image plane, said lens assembly comprising a plurality of linear lens arrays, each lens array associated with a specific magnification value, each lens array formed as an integral part of said lens assembly and means for incrementally illuminating a document and for directing the reflected image into a selected one of the lens arrays whereby the illuminated portion of the document is projected onto the photosensitive image plane at the magnification value of the selected lens array. In a first embodiment, the lens arrays are integrally formed within the body of a planar glass member. In a second embodiment, the lens arrays are formed within the body of a generally cylindrical, partly opaque, glass body.